Motorola MC68340 manual Function Code Signals, SIZx Signal Encoding

Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.

Table 3-1. SIZx Signal Encoding

SIZ1	SIZ0	Transfer Size
0	1	Byte
1	0	Word
1	1	Three Bytes
0	0	Long Word

3.1.2 Function Code Signals

FC3–FC0 are outputs that indicate one of 16 address spaces to which the address applies. Fifteen of these spaces are designated as either user or supervisor, program or data, and normal or direct memory access (DMA) spaces. One other address space is designated as CPU space to allow the CPU32 to acquire specific control information not normally associated with read or write bus cycles. FC3–FC0 are valid while AS is asserted.

Function codes (see Table 3-2) can be considered as extensions of the 32-bit address that can provide up to 16 different 4-Gbyte address spaces. Function codes are automatically generated by the CPU32 to select address spaces for data and program at both user and supervisor privilege levels, a CPU address space for processor functions, and an alternate master address space. User programs access only their own program and data areas to increase protection of system integrity and can be restricted from accessing other information. The S-bit in the CPU32 status register is set for supervisor accesses and cleared for user accesses to provide differentiation. Refer to 3.4 CPU Space Cycles for more information.

Table 3-2. Address Space Encoding

	Function Code Bits
3	2	1	0	Address Spaces
0	0	0	0	Reserved (Motorola)
0	0	0	1	User Data Space
0	0	1	0	User Program Space
0	0	1	1	Reserved (User )
0	1	0	0	Reserved (Motorola)
0	1	0	1	Supervisor Data Space
0	1	1	0	Supervisor Program Space
0	1	1	1	CPU Space
1	x	x	x	DMA Space

MOTOROLAMC68340 USER’S MANUAL3- 3

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